glXFreeContextEXT frees
the client-side part of a GLXContext that was created with glXImportContext.
glXFreeContextEXT does not free the server-side context information or
the XID associated with the server-side context.

glXFreeContextEXT is part
of the EXT_import_context extension, not part of the core GLX command set.
If GLX_EXT_import_context is included in the string returned by glXQueryExtensionsString,
when called with argument GLX_EXTENSIONS, extension EXT_vertex_array is
supported.